Method and apparatus for activation and de-activation of power conditioners in distributed resource island systems using low voltage ac

ABSTRACT

Method and apparatus for controlling power conditioners in a distributed resource island. In one embodiment, the method comprises comparing, at a power conditioner operating in a de-energized state, an input of the power conditioner to an input threshold, wherein the power conditioner is coupled to an islanded grid; operating the power conditioner, when the input exceeds the input threshold, in a soft-grid mode to generate a touch-safe AC voltage that is coupled to the islanded grid; comparing, by the power conditioner, an impedance of the islanded grid to a grid impedance range; comparing, by the power conditioner, load demand of the power conditioner to an activate threshold; and activating the power conditioner, when the impedance is within the grid impedance range and the load demand exceeds the activate threshold, to operate proximate its nominal output voltage.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.15/047,337 entitled “Method and Apparatus for Activation andDe-Activation of Power Conditioners in Distributed Resource IslandSystems Using Low Voltage AC” and filed Feb. 18, 2016, which claimsbenefit of U.S. provisional patent application Ser. No. 62/117,543entitled “Black-Start of Distributed Resource Island System Using LowVoltage AC” and filed Feb. 18, 2015, and U.S. provisional patentapplication Ser. No. 62/255,782 entitled “Activation and De-activationof Inverters In Distributed Resource Island Systems Using Low VoltageAC” and filed Nov. 16, 2015. Each of the aforementioned patentapplications is herein incorporated in its entirety by reference.

BACKGROUND OF THE INVENTION Field of the Invention

Embodiments of the present disclosure relate generally to distributedpower generation systems and, more particularly, to a black-start of adistributed power generation system.

Description of the Related Art

In an Electric Power System (EPS), a black-start is the process ofrestoring power after an outage. In the case of a distributed resourceisland system, coordination is required between distributed energyresources (DERs) and a central controller. In a microgrid, a microgridinterconnect device (MID) is used for disconnecting (i.e., islanding)the system from the main grid or a larger microgrid. The centralcontroller (which is the role of the MID) ensures that the island systemis disconnected from the main grid or larger microgrid prior toblack-start in order to prevent danger to line-workers. In addition,distributed energy resources need to receive a central command from acontroller in order to prevent unwanted energization; for example,during installation or maintenance operations.

During a power outage, the central controller loses access to AC powerand requires some form of energy storage or generation source, such as asuper-capacitor, battery, or generator, to stay powered. However, suchcomponents undesirably add to overall system cost and adversely impactthe system reliability.

Therefore, there is a need in the art for efficiently controlling powerconditioner activation and de-activation in a distributed resourceisland system (DRIS).

SUMMARY OF THE INVENTION

Embodiments of the present invention generally relate to a method andapparatus for controlling power conditioners in a distributed resourceisland system substantially as shown in and/or described in connectionwith at least one of the figures, as set forth more completely in theclaims.

These and other features and advantages of the present disclosure may beappreciated from a review of the following detailed description of thepresent disclosure, along with the accompanying figures in which likereference numerals refer to like parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentinvention can be understood in detail, a more particular description ofthe invention, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this invention and are therefore not to beconsidered limiting of its scope, for the invention may admit to otherequally effective embodiments.

FIG. 1 is a block diagram of a system for power conversion using one ormore embodiments of the present invention;

FIG. 2 is a flowchart depicting a method for power conversion employinga black-start capability in accordance with one or more embodiments ofthe present invention;

FIG. 3 is a block diagram of a power conditioner controller inaccordance with one or more embodiments of the present invention; and

FIG. 4 is a block diagram of a controller for the system controller inaccordance with one or more embodiments of the present invention.

DETAILED DESCRIPTION

FIG. 1 is a block diagram of a system 100 for power conversion using oneor more embodiments of the present invention. This diagram only portraysone variation of the myriad of possible system configurations anddevices that may utilize the present invention.

The system 100, which may also be referred to as a microgrid oradistributed resource island system (DRIS), is a distributed powergeneration system that comprises a plurality of power conditioners102-1, . . . 102-N, . . . 102-N+M, collectively referred to as powerconditioners 102; a plurality of DC power sources 104-1 . . . 104-N,collectively referred to as DC power sources 104; a plurality of energystorage/delivery devices (e.g., batteries) 112-1 . . . 112-M,collectively referred to as energy storage/delivery devices 112; asystem controller 106; a bus 108; and a load center 110. The DC powersources 104 may be any suitable DC source, such as an output from aprevious power conversion stage, a battery, a renewable energy source(e.g., a solar panel or photovoltaic (PV) module, a wind turbine, ahydroelectric system, or similar renewable energy source), or the like,for providing DC power. In some embodiments, such as the embodimentsdescribed with respect to FIG. 1, the DC sources 104 are PV modules andare referred to as “PV modules 104”. Although the energystorage/delivery devices 112 may be any suitable device that can storeenergy and deliver the stored energy for use (e.g., batteries,hydroelectric dams, or the like), in the embodiments described hereinwith respect to FIG. 1 the energy storage/delivery devices 112 arebatteries and are referred to herein as “batteries 112”.

The power conditioners 102 are bi-directional DC:AC inverters, althoughin other embodiments the power conditioners 102 may be other types ofpower converters (e.g. AC-AC matrix converters).

Each power conditioner 102-1 . . . 102-N is coupled to a PV module 104-1. . . 104-N, respectively, in a one-to-one correspondence, although insome other embodiments multiple PV modules 104 may be coupled to one ormore of the power conditioners 102. Each power conditioner 102-(N+1) . .. 102-(N+M) is coupled to a battery 112-1 . . . 112-(M), respectively,in a one-to-one correspondence, where each pairing of a powerconditioner 102 and a battery 112 forms an “AC battery” 130 (e.g., thebattery 112-1 and the power conditioner 102-(N+1) form the AC battery130-1, and the battery 112-M and the power conditioner 102-(N+M) formthe AC battery 130-M). The power conditioners 102 are coupled to thesystem controller 106 via the bus 108; the bus 108 is further coupled toone or more loads and a power grid 160 (e.g., a commercial grid, one ormore other microgrids, or the like) via the load center 110.

When the system 100 is operating in a normal mode (i.e., while connectedto the power grid 160) and the system controller 106 is receiving poweron the AC bus 108 to operate, the system controller 106 is capable ofcommunicating with the power conditioners 102 by wireless and/or wiredcommunication (e.g., power line communication (PLC)) for providingoperative control and/or monitoring of the power conditioners 102 (e.g.,communicating commands to the power conditioners 102, obtaining dataregarding the performance of the power conditioners 102, and the like).In some embodiments, the system controller 106 may be a gateway forreceiving information from and/or sending information to another device(such as a remote master controller, not pictured) via a communicationsnetwork, for example via the Internet. In such embodiments, the systemcontroller 106 may communicate information pertaining to the powerconditioners 102 (e.g., performance data) to the remote mastercontroller, and/or communicate data from the remote master controller(e.g., control commands) to one or more of the power conditioners 102.

The system 100 further comprises a microgrid interconnect device (MID)140, which may also be referred to as an island interconnect device(IID), for determining when to disconnect from/connect to the power grid160, and for performing the disconnection/connection. For example, theMID 140 may detect a grid fluctuation, disturbance or outage and, as aresult, disconnect the system 100 from the power grid 160. Oncedisconnected from the power grid 160, the system 100 can continue togenerate power as an intentional island without imposing safety risks onany line workers that may be working on the power grid 160. The MID 140comprises a disconnect component (e.g., a contactor or the like) forphysically disconnecting/connecting the system 100 to the grid or alarger microgrid. In some embodiments, the MID 140 may additionallycomprise an autoformer for coupling the balanced power system 100 to asplit-phase load that may have a misbalance in it with some neutralcurrent.

In some alternative embodiments, the system controller 106 comprises theMID 140 or a portion of the MID 140. For example, the system controller106 may comprise an islanding module for monitoring the power grid 160,detecting grid failures and disturbances, determining when to disconnectfrom/connect to the power grid 160, and driving a disconnect componentaccordingly, where the disconnect component may be part of the systemcontroller 106 or, alternatively, separate from the system controller106. In other embodiments, such as the embodiment depicted in FIG. 1,the MID 140 is separate from the system controller 106 and comprises adisconnect component as well as a CPU and an islanding module formonitoring the power grid 160, detecting grid failures and disturbances,determining when to disconnect from/connect to the power grid 160, anddriving the disconnect component accordingly. In some embodiments, theMID 140 may coordinate with the system controller 106, e.g., using powerline communications. Thus, the disconnection/connection of the system100 to the power grid 160 is a controlled process driven by the MID 140.

The power conditioners 102-1 . . . 102-N convert the DC power from thePV modules 104 to AC output power and couple the generated output powerto the load center 110 via the bus 108. The load center 110 is furthercoupled to the power 160 grid (e.g., the commercial power grid, a largermicrogrid, or the like) as well as to one or more loads (e.g.,electrical appliances). The power conditioners 102-(N+1) . . . 102-(N+M)are bidirectional converters that can convert AC from the AC bus 108 toDC and store the resulting energy in the corresponding batteries 112-1). . . 112-M), and can convert DC from the corresponding batteries 112-1). . . 112-M) to AC and couple the generated AC output to the AC bus 108.As such, the system 100 can generate AC output power during daylighthours via the PV modules 104, store at least a portion of the generatedenergy in the batteries 112, and continue to generate AC output duringthe evening hours using the energy stored in the batteries 112. Whileconnected to the power grid 160, the power conditioners 102 may operatein a “power mode” where the grid voltage is used as a reference voltagefor synchronizing the output from the power conditioners 102.

In accordance with one or more embodiments of the present invention,each of the power conditioners 102-1 . . . 102-(N+M) comprises acorresponding controller 116-1 . . . 116-(N+M) having a black-startmodule 114-1 . . . 116-(N+M) for providing a black-start to startoperation of the system 100 when it is islanded and de-energized. Thesystem 100 may be islanded (i.e., not connected to the power grid 160)for a variety of reasons; for example, the MID 140 may disconnect thesystem 100 from the power grid 160 following a power failure on thepower grid 160, a user may manually disconnect the system 100 from thepower grid 160 to perform maintenance and/or add components, the system100 may have been installed and not yet connected to the power grid 160,or the like. In some embodiments, the system 100 may be de-energizedwhen islanded as a result of its load exceeding the system generationcapability. In other embodiments, the system 100 may be de-energizedwhen islanded as a result of receiving insufficient input from thesources 104 and 112; for example, in one or more embodiments where theDC sources 104 are PV modules, the system 100 may become de-energizedwhile islanded during the night when the PV modules are not producingany output and if the batteries 112 have insufficient storage to meetthe load demand. In still other embodiments, the system 100 may beintentionally de-energized while islanded to conduct routine maintenanceor to expand the system 100. Prior to the black-start, one or more loads(such as non-critical loads) may be disconnected from the system 100 asnecessary.

While the system 100 is islanded, those power conditioners 102 havingsufficient DC input begin initiating a black-start and generate atouch-safe AC voltage (i.e., a low-level AC voltage on the AC bus 108)that allows communication via the power line (if needed) and allowspreliminary electrical testing to be performed as needed (for example,monitoring of the islanded grid impedance by the power conditioners102). In some embodiments, the touch-safe voltage powers the systemcontroller 106 sufficiently for the system controller 106 to coordinateoperation of the system 100; in other embodiments, the powerconditioners 102 self-initiate start-up and shut-down without requiringexternal communication as described further below.

When energy is stored in the batteries 112-1 . . . 112-M, theircorresponding power conditioners 102-(N+1) . . . 102-(N+M) can receivesufficient power from the batteries 112-1 . . . 112-M for the powerconditioner electronics to remain active. Additionally, when the PVmodules 104 receive sufficient sunlight, their corresponding powerconditioners 102-1 . . . 102-N can receive sufficient power from the PVmodules 104 for their power conditioner electronics to remain active. Assuch, those power conditioners 102 that have sufficient DC input eachgenerate the touch-safe AC voltage that is coupled to the AC bus 108,resulting in a low-voltage AC bus. The touch-safe AC voltage is lowenough to pose no safety risk while providing sufficient power for thesystem controller 106 to restart and issue commands for starting thepower conditioners 102 and/or the power conditioners 102 toself-initiate start-up. In some embodiments, the touch-safe voltage maybe on the order of 30V peak or less, although in other embodiments thetouch-safe voltage may be greater than 30V. In some alternativeembodiments, those power conditioners 102 generating the touch-safevoltage may use one or more communication techniques (e.g., power linecommunications) for communicating with one another.

In one or more embodiments where the system 100 has been de-energizedfor a long duration, one or more of the batteries 112 may be used inassisting the black-start. In such embodiments, the batteries 112 mustfirst be charged to allow the system black-start. The batteries 112 maybe charged at rated current in a soft-grid mode described below. Thischarging will then allow a minimum state of charge to be reached toallow the black-start; once the system is energized to nominal voltagethe batteries 112 can subsequently charge at full power from thecorresponding power conditioners 102.

In those embodiments where the system controller 106 initiates systemstart-up, the system controller 106 remains powered by the touch-safevoltage on the AC bus 108 from one or more of the power conditioners 102and can determine the state of the system 100 (e.g., whether the system100 is being installed or serviced and should not be started, or whetherthe system 100 should restart to operate as a microgrid) and begincoordinating operation of the power conditioners 102 as needed. In someembodiments, the system controller 106 may receive and aggregateinformation from those power conditioners 102 that are participating ingenerating the touch-safe voltage for determining the system state. Whenthe system controller 106 determines that the power conditioners 102should resume operation, the system controller 106 provides theappropriate commands to the power conditioners 102 to begin full-voltageoperation. Those power conditioners 102 receiving sufficient input powercan then begin to operate the AC bus 108 at a nominal grid voltage, andthe system controller 106 can then continue to be powered from thevoltage on the AC bus 108.

In some other embodiments, rather than all of the sufficiently-poweredpower conditioners 102 generating the touch-safe voltage, a subset ofthose power conditioners 102 receiving sufficient input power areconfigured to generate the touch-safe voltage. At system restart, afirst subset of the sufficiently-powered power conditioners 102 generatethe touch-safe voltage which results in a peak value at or below anyapplicable regulatory limit or shock hazard threshold. For example, insome embodiments the touch-safe voltage in on the order of 30V peak orless. The touch-safe voltage provides sufficient power for the systemcontroller 106 to start, monitor the power grid 160 for restoration ofpower, and, when safe for the system 100 to operate as a microgrid,provide the commands to the first subset of power conditioners 102 tobegin full-voltage operation. The remaining sufficiently-powered powerconditioners 102 may be de-activated, for example by de-energization ofa contactor (not shown) positioned strategically along AC bus 108.

In those embodiments where the islanded power conditioners 102self-initiate start-up and shut-down without having to receive anyexternal communication, such as commands from the system controller 106,each power conditioner 102 enters into an “idle” mode (which may also bereferred to as “de-energized”) when it initially starts-up during whichthe power conditioner 102 monitors its DC input and its output power isdisabled. Upon detecting sufficient DC input (i.e., when the DC inputmeets or exceeds an input threshold), the power conditioner 102 switchesinto a “soft-grid” mode where it outputs the touch-safe voltage. Thepower conditioners 102 monitor their load demand; if the load demand issufficiently high (e.g., if the amount of power or the amount of currentthat the power conditioner 102 would need to generate exceeds anactivate threshold), the power conditioner 102 ramps-up to generate therequired output power and the AC bus 108 is operated at a nominal gridvoltage. Generally, the algorithm could be applied to the active outputcurrent, the reactive output current or some combination of the two. Incertain embodiments, the absolute value of the inverter's requiredoutput current may be compared to the activate threshold so that thefirst power conditioner 102 to start up will engage the remaining powerconditioners 102 by pushing their active currents negative. This couldalso be done with a separate negative current threshold or by monitoringthe amplitude of the voltage and starting up when an increase isdetected.

The power conditioners 102 continues to operate at or proximate theirrated (i.e., nominal) voltage until their load demand drops below astandby threshold; in some embodiments, the power conditioners 102 maycompare their required output power or required output current to thestandby threshold. When the load demand for a power conditioner 102drops below the standby threshold (for example, 5% of a powerconditioner's rated power), the power conditioner 102 enters a standbymode where it continues to run its controller 116 and monitor the AC bus108 to determine what power it would be running at if it were active, aswell as the state of the islanded grid. If the load demand subsequentlyrises to or above a re-activate threshold, the power conditioner 102begins operating again at or proximate its rated (nominal) voltage; ifthe AC bus 108 drops below a low grid threshold, the power conditioner102 switches back into its soft-grid mode.

In some embodiments, the activate and re-activate thresholds for thepower conditioners 102 may be set to the same value, for example 10% ofthe power conditioner's rated power, 0.3 amps, or the like, In otherembodiments the activate and re-activate thresholds may be set differentvalues.

In those embodiments where the power conditioners 102 self-initiatestart-up and shut-down as described herein, commissioning anddecommissioning of the system 100 may occur from the load center 110using breakers and/or switches. For example, by opening a breaker orswitch that feeds one or more of the power conditioners 102 (e.g., abranch of the power conditioners 102), those power conditioners 102 willnaturally fall beneath their standby thresholds, their outputs will allde-energize, and they will then switch into the soft-grid mode ofoperation. When a load is subsequently connected to those powerconditioners 102 such that their load demand exceeds the re-activatethreshold, the power conditioners 102 having sufficient DC input willramp-up to operate at or proximate their rated (i.e., nominal) voltage.As one example, the commissioning process to start a typical residentialsystem (e.g., the system 100) may include the following steps: (1) startwith all breakers at the load center 110 open; (2) close all powerconditioner breakers at the load center 110, thereby causing all powerconditioners 102 having sufficient DC input to synchronize in theirsoft-grid modes so there are no hazardous voltages on the AC bus 108;and (3) close all load breakers at the load center 110. Subsequently,the system 100 may be decommissioned by opening all breakers at the loadcenter 110.

Further, by operating the system 100 such that the power conditioners102 self-initiate start-up and shut-down as described herein, the powerconditioners 102 can be rapidly shut-down, without having to rely on anycommunication to the power conditioners 102, by disconnecting them fromthe load. For example, when the power conditioners 102 are disconnectedfrom the load through a shunt-trip breaker or other means, they will alldrop below standby threshold and as a result will de-activate. When allof the power conditioners 102 are disconnected and shut down, the powerconditioners 102 “see” that the there is no longer any voltage on theislanded grid and re-start in the soft-grid mode.

In some embodiments, the power conditioners 102 may require a signalfrom a central controller (e.g., the MID 140) before they can transitioninto the ‘nominal grid mode’. This signal could be communicated from thecentral controller by wired (e.g., PLC) and/or wireless techniques. Thesignal could also be communicated by another device, external to thecentral controller, such as a 17 Hz tone injection unit.

Additionally, in those embodiments where the power conditioners 102self-initiate start-up and shut-down as described herein, the system 100when islanded can operate as a microgrid more efficiently under lightload conditions. For example, in one embodiment where the system 100comprises thirty power conditioners 102 capable of delivering 300 Weach, the standby threshold is set to 20% of the maximum power of thepower conditioners 102. As the system load drops below 0.2*30*300=1.8kW, the power conditioners 102 begin to drop below their standbythresholds, but not at exactly the same time. As the first of the powerconditioners 102 shuts down, the remaining power conditioners 102 areforced to pick up the load and run at higher powers. Such controlnaturally deactivates one or more of the power conditioners 102 to keepthe active power conditioners 102 over the standby threshold, which canbe programmed to maintain a minimum system efficiency. Those powerconditioners 102 that standby continue to monitor the islanded grid, runtheir control (e.g., droop control or the like) to determine what powerthey would be running at if activated, and, if the islanded grid has notbecome de-energized, re-activate when they detect their required outputpower is above the re-activate threshold, for example 25% of theirmaximum rated power.

In one or more of the embodiments described above, the islanded powerconditioners 102 operate in a “voltage control mode” where a droopcontrol technique is employed for parallel operation of the powerconditioners 102 without the need for any common control circuitry orcommunication between the power conditioners 102. One example of such adroop control technique may be found in commonly assigned, U.S.provisional patent application having attorney docket number EE166P andentitled “Time-domain Droop Control with Integrated Phasor CurrentLimiting”, which is herein incorporated in its entirety by reference.

Once a connection to the power grid 160 has been restored (via the MID140), the power conditioners 102 may resume operating in the power mode;i.e., the system controller 106, as coordinated with or part of the MID140, may issue a command to the power conditioners 102 to restart in thepower mode. In some embodiments, the power mode may be a subset of thevoltage control mode, while in other embodiments it is not.

The invention described herein thus enables a black-start capabilitythat is necessary in the event that a seamless transition betweengrid-tied operation and non-grid-tied operation of the system 100 cannotoccur.

FIG. 2 is a flowchart depicting a method 200 for power conversionemploying a black-start capability according to one or more embodimentsof the present invention. The power conversion is performed by a powerconditioner that is part of an islanded microgrid; for example, an MIDmay island the microgrid after detecting a grid failure, or themicrogrid may be completely separate from another power grid (e.g., acommercial power grid or another microgrid) and operate as anindependent microgrid. The microgrid comprises a plurality of powerconditioners coupled, via an AC bus, to a system controller, a loadcenter, and an MID. The microgrid is further coupled to one or moreloads via the load center.

The power conditioner receives input power from an input power sourceand generates an output power. In some embodiments, the powerconditioner may be a bidirectional conditioner that can convert DC-ACand AC-DC. The power conditioner, along with other analogous powerconditioner, is coupled to an AC bus that is coupled to one or moreloads (e.g., via a load center). In some embodiments, the powerconditioner is a power conditioner 102 of the system 100 previouslydescribed, and the method 200 is an implementation of the black-startmodule 114 described further below with respect to FIG. 3. In otherembodiments, the microgrid is completely separate from another powergrid (e.g., a commercial power grid, another microgrid, or the like) andoperates as an independent microgrid.

The method 200 is entered at start block 202 and proceeds to block 204,where the power conditioner is in a de-energized state. In someembodiments, the power conditioner may be de-energized due to themicrogrid being de-energized as a result of the local load exceeding themicrogrid generation capability. In other embodiments, the microgridpower conditioners may be de-energized as a result of receivinginsufficient input from their corresponding power sources. For example,in one or more embodiments where the power sources are PV modules, themicrogrid may become de-energized during the night if the microgrid hasinsufficient storage to meet load demands during the night. In stillother embodiments, the microgrid may be intentionally powered-down toconduct routine maintenance or to expand the microgrid (e.g., by addingone or more resources). The method 200 proceeds from step 204 to step206.

At step 206, a determination is made whether the power conditioner hassufficient input voltage to operate in the soft-grid mode where thetouch-safe low voltage output is generated. For example, the powerconditioner input voltage may be compared to an input threshold formaking the determination. In some embodiments, the power conditioner maybe coupled to a PV module that receives enough sunlight during the dayto generate a sufficient amount of DC input to the power conditioner butat night does not, or the power conditioner may be coupled to a batterythat may or may not have enough stored energy to provide sufficientinput voltage to the power conditioner. In some alternative embodiments,the input current and/or the input power to the power conditioner may beevaluated for determining whether to operate in the soft-grid mode.

If the result of the determination at step 206 is no, that there is notsufficient input voltage (e.g., if the input voltage is less than theinput threshold), the method 200 returns to step 204. If the result ofthe determination at step 206 is yes, that there is sufficient inputvoltage (e.g., if the input voltage meets or exceeds the inputthreshold), the method 200 proceeds to step 208.

At step 208, the power conditioner operates in a soft-grid mode. In thesoft-grid mode, the power conditioner generates a touch-safe voltage andcouples the touch-safe voltage to the AC bus. In some embodiments, thetouch-safe voltage may be on the order of 30V peak or less; in otherembodiments, the touch-safe voltage may be greater than 30V peak.

At step 210, a determination is made whether the grid impedance iswithin an acceptable range. When the grid impedance is within theacceptable range, the islanded grid is considered to be operable; whenthe grid impedance is outside of the acceptable range, the islanded gridis considered to be inoperable. The grid impedance as determined by apower conditioner may be compared to a grid impedance range defined byone or more grid impedance thresholds for determining whether the gridimpedance is within the acceptable range. For example, the gridimpedance may be compared to an upper impedance threshold and to a lowerimpedance threshold, where the grid impedance is considered to be withinthe acceptable range when it is between the upper and the lowerimpedance thresholds.

If the result of the determination is no, that the grid impedance in notwithin the acceptable range, the method 200 returns to step 208. In oneor more embodiments, the grid impedance may be compared to a pluralityof grid impedance ranges for determining whether to return to step 208or, alternately, return to step 204 (as indicated by the dashed line tostep 204). For example, the grid impedance may be compared to a firstgrid impedance range and to a second grid impedance range, where thegrid impedance outside of the first range indicates a potential safetyhazard with respect to the grid and the power conditioner returns to thede-energized state at step 204, and the grid impedance outside of thesecond range indicates no potential safety hazard with respect to thegrid and the power conditioner returns to the soft-grid mode in step208.

If the result of the determination at step 210 is yes, the method 200proceeds to step 212. In some embodiments, the power conditioner mayrequire a signal from a central controller (e.g., an MID) beforeproceeding to step 212. This signal could be communicated from thecentral controller by wired (e.g., PLC) and/or wireless techniques. Thesignal could also be communicated by another device, external to thecentral controller, such as a 17 Hz tone injection unit

At step 212, a determination is made whether the load demand exceeds anactivate threshold; for example, the amount of power or the amount ofcurrent that the power conditioner would need to generate to meet theload demand is compared to the activate threshold. In some embodiments,the activate threshold may be set at 10% of the power conditioner'srated power; in other embodiments, the activate threshold may be set at0.3 amps. If the result of the determination is no, the method 200returns to step 208 and continues to operate in the soft-grid mode. Ifthe result of the determination is yes, the method 200 proceeds to step214.

At step 214, the power conditioner ramps-up to operate at or proximateits rated (i.e., nominal) voltage such that the islanded grid isoperated at a nominal grid voltage. The method 200 proceeds to step 216where a determination is made whether the grid impedance is within theacceptable range. If the result of the determination is yes, the method200 proceeds to step 218. If the result of the determination is no, themethod 200 returns to step 204 where the power conditioner is in ade-energized state.

At step 218, a determination is made whether the load demand exceeds astandby threshold; for example, the power conditioner may compare itsrequired output power or required output current to the standbythreshold to determine whether the load demand exceeds the standbythreshold. In some embodiments, the standby threshold may be on theorder of 5% of the power conditioner's rated power. If the result of thedetermination is yes, the method 200 returns to step 214 where the powerconditioner continues to operate at its nominal voltage. If the resultof the determination is no, the method 200 proceeds to step 220.

At step 220, the power conditioner switches to a standby mode. One ormore other power conditioners in the microgrid may, however, remainoperating at their nominal voltage based on their load demand.

The method 200 proceeds to step 222 where a determination is madewhether the grid voltage is below a low grid threshold; for example, thepower conditioner determines the grid voltage and compares it to the lowgrid threshold. If the result of the determination is yes, the method220 returns to step 204 where the power conditioner is de-energized. Ifthe result of the determination is no, the method 200 proceeds to step224.

At step 224 a determination is made whether the load demand exceeds are-activate threshold; for example, the power conditioner may compareits required output power or required output current to the re-activatethreshold to determine whether the load demand exceeds the re-activatethreshold. In some embodiments, the activate and re-activate thresholdsfor the power conditioners 102 may be set to the same value, for example10% of the power conditioner's rated power, 0.3 amps, or the like. Inother embodiments the activate and re-activate thresholds may be setdifferent values. If the result of the determination is no, the method200 returns to step 220 where the power conditioner remains in thestandby mode. If the result of the determination is yes, the method 200proceeds to step 226, where a determination is made whether to continue.If the result of the determination is yes, the method 200 returns tostep 214 where the power conditioner operates at its nominal voltage.If, at step 226, the result of the determination is no, the method 200proceeds to step 228 where it ends.

FIG. 3 is a block diagram of a power conditioner controller 116 inaccordance with one or more embodiments of the present invention. Thecontroller 116 comprises support circuits 304 and a memory 306, eachcoupled to a central processing unit (CPU) 302. The CPU 302 may compriseone or more conventionally available microprocessors ormicrocontrollers; alternatively, the CPU 302 may include one or moreapplication specific integrated circuits (ASICs). The support circuits304 are well known circuits used to promote functionality of the CPU302. Such circuits include, but are not limited to, a cache, powersupplies, clock circuits, buses, input/output (I/O) circuits, and thelike. The controller 116 may be implemented using a general purposecomputer that, when executing particular software, becomes a specificpurpose computer for performing various embodiments of the presentinvention. In other embodiments, the CPU 302 may be a microcontrollercomprising internal memory for storing controller firmware that, whenexecuted, provides the controller functionality described herein.

The memory 306 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 306 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory306 generally stores the operating system (OS) 308, if necessary, of thecontroller 116 that can be supported by the CPU capabilities. In someembodiments, the OS 308 may be one of a number of commercially availableoperating systems such as, but not limited to, LINUX, Real-TimeOperating System (RTOS), and the like.

The memory 306 stores various forms of application software, such as anpower conditioner control module 310 for controlling operation of thepower conditioner 102 (e.g., power conversion (DC-AC conversion andAC-DC conversion), communication, and the like) and a black-start module114 for controlling the power conditioner black-start functionality asdescribed herein. The memory 306 may additionally store a database 312for storing data related to the operation of the power conditioner 102and/or the present invention, such as one or more thresholds describedherein.

FIG. 4 is a block diagram of a controller 118 for the system controller106 in accordance with one or more embodiments of the present invention.The controller 118 comprises support circuits 404 and a memory 406, eachcoupled to a central processing unit (CPU) 402. The CPU 402 may compriseone or more conventionally available microprocessors ormicrocontrollers; alternatively, the CPU 402 may include one or moreapplication specific integrated circuits (ASICs). The support circuits404 are well known circuits used to promote functionality of the CPU402. Such circuits include, but are not limited to, a cache, powersupplies, clock circuits, buses, input/output (I/O) circuits, and thelike. The controller 118 may be implemented using a general purposecomputer that, when executing particular software, becomes a specificpurpose computer for performing various embodiments of the presentinvention. In other embodiments, the CPU 402 may be a microcontrollercomprising internal memory for storing controller firmware that, whenexecuted, provides the controller functionality described herein.

The memory 406 may comprise random access memory, read only memory,removable disk memory, flash memory, and various combinations of thesetypes of memory. The memory 406 is sometimes referred to as main memoryand may, in part, be used as cache memory or buffer memory. The memory306 generally stores the operating system (OS) 408, if necessary, of thecontroller 118 that can be supported by the CPU capabilities. In someembodiments, the OS 408 may be one of a number of commercially availableoperating systems such as, but not limited to, LINUX, Real-TimeOperating System (RTOS), and the like.

The memory 406 stores various forms of application software, such as asystem controller control module 410 for controlling operation of thesystem controller 106 and a system controller black-start module 120 forcontrolling the system controller black-start functionality as describedherein. The memory 406 may additionally store a database 412 for storingdata related to the operation of the system controller 106 and/or thepresent invention, such as one or more thresholds described herein.

In certain embodiments, the memory 406 may additionally store anislanding module for monitoring the grid, detecting grid failures anddisturbances, determining when to disconnect from/connect to the grid,and driving a disconnect component (e.g., an MID) accordingly, where thedisconnect component may be part of the system controller 106 or,alternatively, separate from the system controller 106.

While the foregoing is directed to embodiments of the present invention,other and further embodiments of the invention may be devised withoutdeparting from the basic scope thereof, and the scope thereof is definedby the claims that follow.

1. A method for controlling power conditioners in a distributed resourceisland system (DRIS), comprising: comparing, at a power conditioneroperating in a de-energized state, an input of the power conditioner toan input threshold, wherein the power conditioner is coupled to anislanded grid; operating the power conditioner, when the input exceedsthe input threshold, in a soft-grid mode to generate a touch-safe ACvoltage that is coupled to the islanded grid; comparing, by the powerconditioner, an impedance of the islanded grid to a grid impedancerange; comparing, by the power conditioner, load demand for the powerconditioner to an activate threshold; and activating the powerconditioner, when the impedance is within the grid impedance range andthe load demand exceeds the activate threshold, to operate proximate itsnominal output voltage.
 2. The method of claim 1, further comprising,subsequent to activating the power conditioner, comparing, by the powerconditioner, the load demand to a standby threshold; and operating thepower conditioner in a standby mode when the load demand is less thanthe standby threshold.
 3. The method of claim 1, further comprising,subsequent to activating the power conditioner, comparing, by the powerconditioner, the impedance to the grid impedance range; and switchingthe power conditioner to a de-energized state when the impedance isoutside of the grid impedance range.
 4. The method of claim 2, furthercomprising, subsequent to operating the power conditioner in the standbymode, comparing, by the power conditioner, the load demand to are-activate threshold; and activating the power conditioner, when theload demand exceeds the re-activate threshold, to operate proximate thenominal output voltage.
 5. The method of claim 1, wherein comparing theload demand to the standby threshold comprises comparing a requiredoutput current of the power conditioner to the standby threshold.
 6. Themethod of claim 1, wherein the power conditioner is a DC-AC powerconditioner.
 7. The method of claim 6, wherein the power conditioner iscoupled to a photovoltaic (PV) module that provides the input.
 8. Anapparatus for controlling power conditioners in a distributed resourceisland system (DRIS), comprising: a power conditioner, coupled to anislanded grid, comprising a black-start module for (i) comparing, whilethe power conditioner is operating in a de-energized state, an input ofthe power conditioner to an input threshold; (ii) operating the powerconditioner, when the input exceeds the input threshold, in a soft-gridmode to generate a touch-safe AC voltage that is coupled to the islandedgrid; (iii) comparing an impedance of the islanded grid to a gridimpedance range; (iv) comparing load demand for the power conditioner toan activate threshold; and (v) activating the power conditioner, whenthe impedance is within the grid impedance range and the load demandexceeds the activate threshold, to operate proximate its nominal outputvoltage.
 9. The apparatus of claim 8, wherein the black-start modulefurther compares, subsequent to activating the power conditioner, theload demand to a standby threshold; and switches the power conditionerto a standby mode when the load demand is less than the standbythreshold.
 10. The apparatus of claim 8, wherein the black-start modulefurther compares, subsequent to activating the power conditioner, theimpedance to the grid impedance range; and switches the powerconditioner to a to a de-energized state when the impedance is outsideof the grid impedance range.
 11. The apparatus of claim 9, wherein theblack-start module further compares, subsequent to switching the powerconditioner in the standby mode, the load demand to a re-activatethreshold; and activates the power conditioner, when the load demandexceeds the re-activate threshold, to operate proximate the nominaloutput voltage.
 12. The apparatus of claim 8, wherein comparing the loaddemand to the standby threshold comprises comparing a required outputcurrent of the power conditioner to the standby threshold.
 13. Theapparatus of claim 8, wherein the power conditioner is a DC-AC powerconditioner.
 14. The apparatus of claim 13, wherein the powerconditioner is coupled to a photovoltaic (PV) module that provides theinput.
 15. A system for controlling power conditioners in a distributedresource island system (DRIS), comprising: a plurality of power sources;and a plurality of power conditioners, coupled to the plurality of powersources in a one-to-one correspondence and coupled to an islanded grid,wherein each power conditioner of the plurality of power conditionerscomprises a black-start module for (i) comparing, while the powerconditioner is operating in a de-energized state, an input of the powerconditioner to an input threshold; (ii) operating the power conditioner,when the input exceeds the input threshold, in a soft-grid mode togenerate a touch-safe AC voltage that is coupled to the islanded grid;(iii) comparing an impedance of the islanded grid to a grid impedancerange; (iv) comparing load demand for the power conditioner to anactivate threshold; and (v) activating the power conditioner, when theimpedance is within the grid impedance range and the load demand exceedsthe activate threshold, to operate proximate its nominal output voltage.16. The system of claim 15, wherein the black-start module furthercompares, subsequent to activating the power conditioner, the loaddemand to a standby threshold; and switches the power conditioner to astandby mode when the load demand is less than the standby threshold.17. The system of claim 15, wherein the black-start module furthercompares, subsequent to activating the power conditioner, the impedanceto the grid impedance range; and switches the power conditioner to a toa de-energized state when the impedance is outside of the grid impedancerange.
 18. The system of claim 16, wherein the black-start modulefurther compares, subsequent to de-activating the power conditioner, theload demand to a re-activate threshold; and activates the powerconditioner, when the load demand exceeds the re-activate threshold, tooperate proximate the nominal output voltage.
 19. The system of claim15, wherein comparing the load demand to the standby threshold comprisescomparing a required output current of the power conditioner to thestandby threshold.
 20. The system of claim 15, wherein the plurality ofpower conditioners are DC-AC power conditioners, and wherein theplurality of power sources are photovoltaic (PV) modules.